In the past, valve guide reworking operations have been accomplished by using either large and immovable machines requiring the precise mounting of cylinder heads on the machine at an angle in order to produce the proper reworking alignment, or small tools which could be mounted on the cylinder head. The large machines typically include a heavy base on which is mounted a drill press or other boring tool, thus necessitating the mounting of the cylinder head in a precise position to provide the proper alignment with the drill press. The cylinder head mounting on these machines is a tedious operation and one which, of necessity, must be repeated for each valve guide since the intake and exhaust valve guides in modern engines are often formed at opposing angles.
The smaller tools, which could be mounted on the cylinder head when the cylinder head was in any position, were based on alignment principles which often produce inaccurate results. One smaller tool utilized the principle of aligning the guide tool with the valve guide bore by means of the valve spring seat on the side of the cylinder head opposite the valve seat. However, valve spring seats are normally not concentrically located with respect to the valve guide bore, thus causing this method to be inaccurate. Other small tools for the reworking operations necessitated mounting the guide fixture on the cylinder head by means of one of the many holes provided in the cylinder head to mount the head on the engine block. However, if the mounting hole was in any way deformed or displaced, the capability of aligning the tool properly was thereby reduced.
One fixture for reworking valve guides solving the aforementioned problems was disclosed in U.S. Pat. No. 3,764,204 to Kammeraad, issued Oct. 9, 1973, and entitled VALVE GUIDE BORING FIXTURE. The arrangement shown in U.S. Pat. No. 3,764,204 requires a valve seat engaging means 40 and a spring seat engaging means 80 which engage opposite sides of a cylinder head. This is not always convenient, and also can require extra set-up time to repeatedly lift and move the cylinder head between successive valve guides to be reworked. Further, the valve seat engaging means 40 includes a ball swivel guide 42 pivotally mounted on fixture arm 18, an elongated guide/bushing 60 that extends through ball swivel guide 42 in a semipermanent arrangement therein, and a valve seat adapter 70 threadably mounted onto bushing 60. The three-piece construction of guide 42, bushing 60 and valve seat adapter 70 can add complexity and looseness in the valve seat engaging arrangement leading to reduced accuracy of alignment to the particular valve guide being reworked if the arrangement is not properly maintained and adjusted. For example, in practice, some operators have adjusted valve seat adapter 70 to a low position on elongated bushing 60. This causes an unnecessarily long distance between the valve seat 106/108 and the fixture arm 18, which can reduce the accuracy of alignment when arm 18 biases valve seat adapter 70 against the selected valve seat. In particular, the greater the distance between valve seat 108 and ball swivel guide 42, the less stable the arrangement. This can reduce the accuracy of alignment as the fixture flexes during use. Still further, the guide/bushing 60 cannot be removed or replaced without releasing valve seat adapter 70 from valve seat 106/108. For example, it may be desirable to change bushing 60 so that a different diameter boring tool can be used. With the device disclosed, valve seat adapter 70 must be released from engagement with valve seat 108.
At least one fixture for reworking valve guide bores has been manufactured which utilizes a forked arrangement for supporting a valve seat engaging adapter. See Hungary Patent Publication 53832, filed May 16, 1989. The forked arrangement includes a pair of fingers that extend laterally and loosely support the valve seat engaging adapter therebetween, permitting the adapter to both tilt and slide on the fingers as the adapter is biased into engagement with the valve seat. In theory, this allows the adapter to assume an optimal position which is concentrically aligned with the centerline of the valve seat and the valve guide bore. However, in practice, the forces generated by the fingers are not always concentrically balanced with respect to the valve seat centerline. For example, this may occur if the arm supporting the forked arrangement is adjusted too high or too low relative to the valve seat. Hence, the adapter takes on a compromised position that is a complex reaction to forces generated by the valve seat on the valve seat adapter, the frictional forces generated by movement and flexing of the fixture, and the unbalanced forces generated by the forked supporting arrangement. Thus, an improved fixture is desired which solves the aforementioned problems.